The present invention relates to single photon emission computed tomography (SPECT) and more particularly to a system or camera/gantry configuration which reduces the degree of patent support table motion required to position a patient adjacent to gamma cameras for imaging when cameras are positioned such that their axis essentially form an L.
SPECT examinations are carried out by injecting a radiopharmaceutical into the body of a patient to be examined. A radiopharmaceutical is a substance labeled with a radioisotope which emits photons at one or more energy levels. By choosing a compound which accumulates in an organ to be imaged, radiopharmaceutical concentration, and hence radioisotope concentration, can be substantially limited to an organ of interest. The organ to be imaged will be referred to hereinafter as an organ of interest and an energy range which approximates the known energy level will be referred to as the energy range.
While moving through a patient's blood stream the radiopharmaceutical becomes concentrated in the organ of interest. By measuring the number of photons emitted from the organ of interest which are within the marker range, organ characteristics, including irregularities, can be identified.
To measure the number of emitted photons planar gamma cameras are used. After a radiopharmaceutical has become concentrated within an organ of interest, a camera is positioned at an imaging angle with respect to the organ of interest such that the organ is positioned within the camera's field of view FOV. The camera is designed to detect photons traveling along preferred paths within the FOV.
A gamma camera consists of a collimator, a scintillation crystal, a plurality of photo multiplier tubes (PMTs) and a camera processor. The collimator typically includes a rectangular lead block having a width dimension and a length dimension which together define the FOV. Holes in the collimator block define the preferred photon paths. The preferred paths are unidirectional and perpendicular to the front face of the collimator. The collimator blocks emissions toward the crystal along non-preferred paths.
The scintillation crystal is positioned adjacent the collimator on a side opposite the FOV and has an impact surface and an oppositely facing emitter surface. The impact surface defines a two dimensional imaging area A having a length L and a width W. Photons which pass through the collimator are absorbed by the scintillation crystal. The crystal converts gamma photons to light photons each time a gamma photon is absorbed. The amount of light emitted depends on the absorbed photon's energy level.
The PMTs typically include between 37 and 91 PMTs which are arranged in a two dimensional array which is positioned adjacent the emitter surface. Light emitted by the crystal is detected by the PMTs which are in the area adjacent the emitter point. Each PMT which detects light generates an analog intensity signal. The intensity signal is proportional to the amount of light detected. When a single photon is absorbed by the crystal, the emitted light is typically absorbed by several different PMTs such that several PMTs generate intensity signals simultaneously. For the purposes of this explanation all intensity signals caused by a single photon will be collectively referred to as a signal set.
The processor receives each signal set and performs a plurality of calculations on each signal set to determine two characteristics of the corresponding photon. The processor combines the intensity signals of each signal set to identify the energy level of a corresponding photon. Photons having energies within the energy range will be referred to as events. Only signals corresponding to events are used for imaging. The processor also performs a series of calculations in an effort to determine precisely where in the crystal area A an event occurred. The processor uses these locations to create an image of the organ of interest which corresponds to the camera imaging angle.
To create a three dimensional "tomographic" image of the organ of interest, a gamma camera can be used to generate a plurality of images from different imaging angles. To this end, the camera is often mounted to an annular gantry and positioned parallel to, and at an imaging angle about, a rotation axis which passes through the organ of interest. The rotation angle is incremented between views so that the plurality of images are generated. The plurality of images are then used to construct pictures of transaxial slices of the torso section using algorithms and methods that are well known to those skilled in the tomographic arts.
To reduce the time required for generating a plurality of images many SPECT systems are equipped with two or more cameras which can be arranged at different angles with respect to the rotation axis. While many different two camera configurations can be formed, there are two configurations which are most widely used, an "H" configuration or mode and an "L" configuration or mode.
Referring to FIG. 1, in the H mode two cameras 10 and 12 oppose each other such that camera axis 11 and 13 are aligned and intersect an rotation axis 15. The H mode is used to image patients efficiently by acquiring two opposing views at the same time. For example, the H mode can be used to acquire two fixed views such as anterior and posterior views simultaneously. In the alternative, the H mode can be used to acquire a series of views for SPECT over 360.degree. by acquiring images from both detectors over a 180.degree. rotation.
Referring to FIG. 2, in the L mode the two cameras 10, 12 are positioned such that their camera axis 11, 13 intersect at rotation axis 15 forming approximately a 90 degree angle .gamma. hence the term "L mode". In one common L mode the axis form a 90 degree angle .gamma. while in another the axis form an essentially 101 degree angle .gamma.. For the purposes of this explanation, although the invention can be used with any L mode configuration, in the interest of simplification, the invention will be described in the context of a 90 degree L mode unless indicated otherwise.
The L mode is typically used to image the heart. During heart imaging data is typically only collected over 180 degrees of rotation about the left side of the chest. To collect 180 degrees of data using cameras configured in the L mode, the cameras are rotated about the patient through 90 degrees, each camera separately collecting 90 degrees of imaging data for a total of 180 degrees.
Because both the L and the H modes are advantageous many SPECT systems are equipped so that two cameras can be positioned in either the H or the L mode. To this end at least one of the two system cameras is mounted such that it can be independently rotated with respect to the other system camera about the gantry, the independently moveable camera lockable in either the L or the H modes. Systems in which an operator can put the detectors in either L mode or H mode are known within the industry as "variable geometry SPECT cameras".
Because SPECT systems are expensive it is important that each system be designed such that it can be used to image virtually all patients independent of patient size. Generally, the largest section of most patients is through their chest or shoulders and is typically between 200 and 560 mm. To accommodate sections as large as 560 mm most system cameras have a field of view which is at least 540 mm. In this case, a detector enclosure or "tub" is typically over 600 mm wide.
It is well known in the industry that SPECT image quality can be improved by positioning a gamma camera as close as possible to an organ to be imaged during data gathering. To this end most SPECT systems provide some mechanism for changing the position of a patient support table relative to a gamma camera to thereby change the position of a patent on the table relative to the camera. Referring again to FIG. 1, in the H mode, one common way to change the position of a patient with respect to cameras 10 and 12 is to mount each camera 10 and 12 on a radial slide (not illustrated) which allows the cameras to be moved radially inwardly and outwardly relative to axis 15 along the direction indicated by arrow 24.
While radial slides work well in the H mode, camera geometry often prohibits radial camera motion in the L mode. Referring to FIGS. 1 and 2, to move the cameras from the H to the L mode, the cameras are first moved radially outward to a position where adjacent corners 26 and 28 will just touch in L mode. In the present example it will be assumed that this is 304 mm. Then, camera 12 is rotated about axis 15 in a counterclockwise direction toward camera 10 until adjacent edges 26 and 28 contact. In this example, edges 26 and 28 contact when the angle between axis 11 and 13 is 90 degrees. As can be seen in FIG. 2, at this point both cameras 10 and 12 cannot be moved radially inward together as the edges 26 and 28 would collide. In addition, if either camera 10 or 12 were moved inward separately, the moved camera would block a portion of the stationary camera.
Thus, in the L mode other means for changing the position of cameras 10 and 12 with respect to a patient have been developed. To this end, one way to change the position of a patient with respect to cameras 10 and 12 has been to provide an adjustable table 30 (see FIG. 2). Table 30 can move both vertically (along arrow 32) and laterally (along arrow 34) and therefore can move a patient toward both cameras 10 and 12. As cameras 10 and 12 are rotated about axis 15, table 30 moves about in a semicircular path to maintain an organ being imaged in an initial position with respect to cameras 10 and 12.
An alternative way to change the position of a patient with respect to the cameras is to mount the cameras on a gantry 44 (see FIG. 2) and move gantry 44 rather than move table 30 laterally. A configuration facilitating table horizontal and gantry lateral movement can be used together to achieve the same effective patient positioning.
Referring still to FIG. 2, the amount of table movement d required to accommodate the smallest patient having a chest diameter of 200 mm can be determined as follows. In FIG. 2, cameras 10 and 12 are in an L configuration defining a 90.degree. angle .gamma.. In this case the fixed radius of camera rotation about imaging axis 15 is R and the effective radius of rotation required to accommodate the smallest patient (i.e. 200 mm chest diameter) is r. Distance d can be determined by solving the following equation: ##EQU1##
Again, assuming a fixed radius R of 304 mm and a required effective radius of 200 mm, where .gamma. is 90.degree., d is 147 mm.
While adjustable tables as described above have been successfully implemented, tables required to provide necessary patient movement are relatively complex and therefore relatively expensive. This is particularly true where relatively large table movements d are required. In addition, large table movements also require a large gantry aperture as seen from FIG. 2. The gantry ring 44 prevents the patient from coming close to the cameras.
Another solution for changing the position of a patient with respect to cameras configured in the L mode has been to mount cameras to a gantry on radial/lateral slides. Referring again to FIG. 2, as above, radial slides allow cameras 10 and 12 to be moved radially with respect to axis 15. One lateral slide allows camera 10 to be moved laterally along the direction indicated by arrow 14 while another lateral slide allows camera 12 to be moved along the direction indicated by arrow 16. In this case, to move cameras 10 and 12 closer to a patient supported by table 30, camera 10 can be moved laterally away from camera 12 (i.e. in FIG. 2, to the left) and radially inward toward axis 15 while camera 12 is moved lateral away from camera 10 (i.e. in FIG. 2, downward) and radially inward toward axis 15. In effect, cameras 10 and 12 are moved in the directions indicated by arrows 36 and 38, respectively.
Unfortunately, while radial/lateral slides also have been successfully implemented, such dual motion slides, like a highly adjustable table, are also relatively complex and therefore are relatively expensive.
Therefore, it would be advantageous to have a two camera SPECT system which reduces the amount of table movement required to accommodate small patients when cameras are configured in the L mode and which does not require cameras to be mounted on lateral slides. In addition, where an adjustable table is not provided, it would be advantageous to have a SPECT system which reduces the amount of lateral slide movement required to accommodate small patients when cameras are configured in the L mode.